*{
	margin: 0;
	padding: 0;
	box-sizing: border-box;
}
.header{
	width: 1000px;
	margin: 0 auto;
}
.col{
	width:33.333333%;
	float: left;
	height: 100px;
	background: red;
}
clearfix::before,
clearfix::after{
	display: block;
	content: "";
	clear: both;
}
@media(max-width:600px){
	.header{
		width: 100%;
	}
	.col{
		width: 100%;
	}
}